Apparatus for reading or writing optical recording carriers

ABSTRACT

The invention concerns an apparatus for reading or writing optical recording carriers, the apparatus using the differential phase detection process for tracking purposes. This process exhibits shortcomings when optical recording carriers of different pit depths are to be used. The object of the invention is to design an apparatus of this type which can also be used with said optical recording carriers. This object is achieved in that delay members are disposed between the photodetector and phase detector. The apparatus according to the invention is used as a playback or recording apparatus for optical recording carriers, such as CDs, CD-ROMs, CD-Is, CD-Rs, DVDs, DVD-ROMs, DVD-Rs, etc.

The present invention relates to a device for reading from and/orwriting to optical recording media according to the preamble of claim 1.

A device of this type is disclosed in U.S. Pat. No. 4,497,048. In thisknown device, there is provision of a photodetector which is dividedinto four areas. Tracking information is obtained by combining thesignals from two areas in each case to form a summation signal and byascertaining a phase difference between the first and the secondsummation signal such that it is not necessary to acquire the amplitudeof the reproduced signal. This device functions satisfactorily when theoptical recording medium to be read out has a well-defined and constantgeometry of the information-carrying layer. In particular, it isnecessary that the depressions in the information-carrying layer whichserve for information storage, the so-called pits, have a constant depthin relation to the wavelength of the light used for the read-outoperation.

What is disadvantageous with the known device is the fact that onlyslight variations in the geometry of the pits are tolerated, and thatlarger deviations lead to errors in the formation of the trackinginformation. The result of this is that high error rates occur duringthe read-out of the optical recording medium or, in the extreme case,such a read-out becomes completely impossible.

The object of the present invention is to improve the known device tothe extent that it can be used without any problems even for opticalrecording media having a different pit depth or pit geometry.

According to the invention, this is achieved by virtue of the fact thata delay element is assigned to at least one of the areas of thephotodetector. This has the advantage that the areas of thephotodetector can be temporally adjusted relative to one another. Sincethe photodetector receives an interference pattern which is caused bythe pits arranged on the optical recording medium and may vary dependingon the disk geometry and the track geometry (pit depth, etc.), on thetype of disk and on manufacturing tolerances and also on the ageing ofthe laser scanner, the temporal relationship between the detectorsignals also varies. The temporal adjustment of the detector signalsrelative to one another which is possible according to the inventionenables variations of this type to be compensated for. The temporaladjustment can in this case be carried out by means of a single delayelement assigned to an area of the photodetector. However, significantlybetter adjustment is possible if a delay element is assigned to aplurality, or each, of the areas. The greatest flexibility is madepossible if a delay element is assigned to each of the areas. If a delayelement is assigned to all but one of the areas, then all of the areasprovided with a delay element can be adjusted to that area without adelay element. It is also possible to combine the areas into groups, toassign a delay element to each group and to adjust the groups relativeto one another. If an area consists of a plurality of individual areas,then a delay element can be assigned to each of these individual areas.

The delay elements can be set externally, for example during productionof the device or when a device service is being carried out.

The invention provides for the presence of control means for setting thedelay time of the delay element or elements. This has the advantage thatthe delay elements can be adjusted at any time. For example, adjustmentcan be carried out after each insertion of a new optical recordingmedium, at specific time intervals, for example during briefinterruptions in the playback or writing operation of an opticalrecording medium, or at intervals which are suitably defined in adifferent way. One adjustment possibility consists in setting fixeddelay times which are matched to the type of optical recording medium.The type of recording medium is in this case defined by the pit depth orthe pit geometry, the track width or similar characteristic properties.What may be involved here are the known audio CD or new opticalrecording media having a high recording density (so-called DVD). It islikewise possible to vary the delay times in a type of “try-and-errormethod” with regard to the initial settings and to retain the new valuewhen the tracking behaviour becomes better. If the tracking behaviourbecomes worse, the delay times are reset to the preceding setting.

Provision is furthermore made for providing a phase detection means fordetecting the phase angle of the signals emitted by the areas of thephotodetector. This has the advantage that the adjustment is possible inan optimum manner since the exact phase angle is determined and,consequently, the delay times of the delay elements can be set exactly.

A further refinement of the invention makes provision of a circuitarrangement for applying a combination of the output signals of theareas to a phase angle detector. This has the advantage that the phaseangles of the individual signals are evaluated by a single phase angledetector in that the signals of the individual areas of thephotodetector are passed successively to the phase angle detector viathe circuit arrangement. The circuit arrangement, for example amultiplexer, is advantageously installed in the device, with the resultthat the phase angle can be determined internally. Alternatively, thisis advantageous for external determination of the phase angle since nounnecessarily high contact-making outlay is required in this case.

The circuit arrangement is advantageously switched by a control means,for example a microcontroller. According to the invention, it islikewise possible for switching to be effected according to a specifictiming cycle or a clock triggered in a different way.

An advantageous development of the invention consists in the fact thatthe phase angle detector is the phase detector of the device. This hasthe advantage that an additional component is not necessary, and thatfunctional groups of the device which are present in any case areoptimally utilized.

In a development of the invention, provision is made for using amicrocontroller which controls both the circuit arrangement and thesetting of the delay time of the delay elements. This has the advantagethat the control is carried out by means of a component which can beintegrated in a cost-effective manner and can be adapted flexibly tochanged boundary conditions. A logic circuit can also advantageously beused instead of a microcontroller.

An advantageous development of the invention consists in arranging alow-pass filter at the output of the phase detector. This has theadvantage that an averaged DC signal is emitted in this way, therebyenabling automatic setting of the delay time even if the light beamscanning the optical recording medium is not following a track. Thelight pulses impinging on the detector do not in this case have tooriginate from a data. track; they may equally well be the signals whichoccur during random crossing of the tracks. In this way, rapid settingof the delay times is possible before the actual playback or recordingoperation begins. Since the phase detector is in any case not requiredfor tracking during this time, a changeover between the functions oftracking and phase adjustment of the delay elements is also unnecessary.

The adjustment of delay elements of the device according to theinvention which are able to be set is advantageously carried outaccording to the method described in claim 8. This has the advantagethat phase differences caused by component properties are compensatedfor in a simple and cost-effective manner. After the method has beencarried out, the device is provided with optimally adapted settingswhich are retained until, if appropriate, renewed adjustment isnecessary or desirable. Not only can the method described be used duringproduction for the purpose of optimum setting of the device, but it canalso be employed for the purpose of adjustment during operation of thesaid device. The latter case can be implemented either manually, forexample during servicing, or automatically, for example at specific timeintervals.

Provision is made for setting those delay elements which are able to beset to a minimum delay time prior to the first method step of the methodaccording to the invention. This has the advantage that the delayelements of the area with the chronologically last signal do not need tobe altered, and that the maximum possible margin is available for thoseof the other delay elements.

The method according to the invention can also advantageously beemployed when no track is actually being followed. In this case, thelight pulses falling onto the detector originate from randomly crossedtracks. However, this is unimportant for setting the delay times by themethod according to the invention, since the chronological sequence ofthe signals of the individual detector elements in the case of arotating recording medium, but with the track regulating circuit stillopen, is virtually identical to that in the case of a closed regulatingcircuit. Optimum adjustment of the delay times can therefore be carriedout by means of the method according to the invention even before theclosing of the track regulating circuit, and this is something whichsignificantly facilitates the subsequent closing of the track regulatingcircuit since no erroneous signals need be used for this purpose.

Claim 10 describes an embodiment of the method according to theinvention which has the advantage of being particularly simple toexecute. The device according to the invention is used as a playback orrecording device for optical recording media such as, for example, CD,CD-ROM, CD-I, CD-R, DVD, DVD-ROM, DVD-R, etc.

The features cited in the individual claims can also expediently beemployed in combination with one another. Further advantages of thedevice according to the invention and of the method according to theinvention can be inferred from the following description.

In the figures:

FIG. 1 shows a diagrammatic illustration of a device according to theinvention,

FIG. 2 shows a flow diagram relating to the method according to theinvention.

FIG. 1 diagrammatically illustrates the device according to theinvention. A light source 1 generates a light beam which passes througha semi-transparent mirror 2 and an objective lens 3 to fall onto theoptical recording medium 4, from where it is reflected and passesthrough the objective lens 3 to the mirror 2, from where it is deflectedonto the detector 5. The detector 5 is illustrated both in side view (onthe left) and in a plan view rotated through 90° (on the right). Thisillustration reveals that the detector 5 consists of four areas A, B, Cand D.

The optical recording medium 4 is caused to rotate by means of a motorM. Depressions, so-called pits, are arranged in a spiral track on theoptical recording medium 4. The objective lens 3 serves to focus thelight beam onto the optical recording medium 4. For this purpose, itsdistance from the optical recording medium 4 can be adjusted by means ofa drive 6. In order to follow the track, that is to say for the purposeof tracking, the objective lens 3 can be displaced by the drive 6 in theradial direction with regard to the optical recording medium 4. Atracking signal TS is applied to the drive 6 for this purpose.

The output signals of the areas A, B, C and D are fed to delay elements7, 8, 9 and 10, respectively. The delay elements 7, 8, 9 and 10 havedelay times τ_(A), τ_(B), τ_(C), and τ_(D), respectively, which can bevariably set. The delayed signals A′, B′, C′ and D′ are then present atthe output of the delay elements 7, 8, 9 and 10, respectively. The saidsignals are fed to a circuit arrangement 11 whose two outputs 12 and 13are fed to a phase detector 14.

The signals A′ and C′ are additionally fed to a first summer 15, whoseoutput signal is fed to the circuit arrangement 11. The signals B′ andD′ are correspondingly fed to a second summer 16, whose output signal islikewise fed to the circuit arrangement 11. Furthermore, the outputsignals of the first summer 15 and of the second summer 16 are fed to athird summer 17, whose output signal represents the information signalHF. The further processing of the signal HF is not described here ingreater detail since it is carried out in a manner which is known to aperson skilled in the art.

The high-frequency component of the signal present at the output 12 isfed via a capacitor 18 to a converter 19. This converter 19 converts theanalog input signal into a square-wave signal which has two possiblestates. This square-wave signal essentially corresponds to theinformation “light is falling onto the corresponding areas of thephotodetector” or “no light is falling onto these areas”, that is to sayessentially corresponds to the analog input signal. Converters of thistype are also referred to as “data slicers”. The same appliescorrespondingly to the signal present at the output 13, which signalpasses through the capacitor 18′ and the converter 19′. The digitizedsignal emitted by the converters 19, 19′ is fed to a phase comparator20. If the phase angle of these input signals is identical, then theoutput signal of the phase comparator is 0; if a phase shift is present,then the output signal of the phase comparator 20 deviates from 0 in thepositive or negative direction, specifically to a greater extent, thegreater the temporal phase shift of the input signals is. The outputsignal of the phase comparator 20 is filtered by means of a low-passfilter 21, the cut-off frequency of which is approximately 50 kHz in theexemplary embodiment, and, having been amplified by means of anamplifier 22, is output as track error signal TE.

The signal TE is filtered in a further low-pass filter 23, the cut-offfrequency of which is significantly lower, approximately 10 Hz in theexemplary embodiment, and is fed to a microcontroller 24 serving ascontrol means. The microcontroller 24 controls both the circuitarrangement 11 via a line 25 and the delay elements 7 to 10 via a line26. In the exemplary embodiment, the line 25 is a three-bit data line,while the line 26 is designed as a serial data line which transmits acontrol signal having the required resolution for each delay element 7,8, 9 and 10.

In the exemplary embodiment, the circuit arrangement 11 is embodied as amultiplexer which connects two times seven inputs to two outputs. Forthe purpose of tracking, the inputs A+C and B+D each illustrated at thebottom in the drawing are connected to the outputs 12 and 13,respectively.

A deviation of the light beam, which reads out the optical recordingmedium 4, from the track centre causes the diffraction pattern fallingonto the detector 5 to assume an asymmetrical distribution, as a resultof which the signals A+C and B+D differ in terms of their phase angle.The output signal TE deviates from 0 to a greater extent, the greaterthe deviation of the light beam from the track centre and thus the shiftin the phase angles of the signals A+C and B+D relative to one another.A regulator (not illustrated here) determines from this a desiredtracking value, which is forwarded as the signal TS to the drive 6.

The delay elements 7 to 10 are provided in order to enable the signalsemitted by the areas A, B, C and D to be temporally adjusted relative toone another. In this case, it is favourable to adjust thechronologically leading detector signals to the respectivechronologically last signal, with the result that the relative phaseshift with respect to one another becomes 0°. The best signal sourcequality with the least noise is then obtained for the track error signalTE. The method cited for determining the track error signal TE is calledthe “Differential Phase Detection” method (DPD). Since the delays thatoccur of the individual areas A to D of the detector relative to oneanother depend on the track depth and the geometry of the opticalrecording medium as well as on the linear scanning speed, which may varyfor different types of optical recording media 4, the delay elements 7to 10 are adjusted anew each time the optical recording medium ischanged. Starting from the existent phase detector 14 for generating thetrack error signal TE by means of the DPD method from the summationsignals A+C and B+D, the phase detector 14 is likewise used, with littleoutlay, for determining the order of the detector signals that appearfrom the areas A to D and for automatically adjusting the delayelements.

The chronological order of the detector signals of the areas A to Drelative to one another is determined as follows: the signals A, B, Cand D are compared with one another in order to be able to ascertaintheir phase angle with respect to one another. Six combinations arenecessary in order to be able reliably to ascertain the signal appearinglast in chronological order. The combinations are A and B, A and C, Aand D, B and C, B and D and also C and D. The multiplexer of the circuitarrangement 11 enables all of the said comparison combinations to beswitched to the inputs of the phase detector 14, depending on the switchposition. The average DC voltage component of the output signal of thephase detector 14 can be determined with the aid of an additionallow-pass filter 23, which, for example, can also be realized in themicrocontroller 24, for example as an FIR filter. All of the delayelements 7 to 10 are firstly set to the smallest possible delay, that isto say τ_(A)=τ_(B)=τ_(C)=τ_(D)=0. If two input signals having adifferent phase angle are then passed to the phase detector 14 and theaverage DC voltage component at the output thereof is measured, avoltage deviating from zero is produced which is positive or negativedepending on the phase angle. The decision concerning the phase anglecan be made, for example, by means of a software comparator, thecomparison level of which is 0 volts. Since it is initially intended todetermine only the chronological order of the detector signals of theareas A to D by alternate interrogation, the interrogation can becarried out very rapidly. It is not necessary to wait for the settlingof the low-pass filter 23. It is only necessary to identify the tendency(greater than or less than 0). After the chronologically last signal hasbeen determined by means of alternate interrogation in a suitable form,by software or by a state machine, in the microcontroller 24 in theexemplary embodiment, the actual adjustment of the delay elements 7 to10 is carried out.

This adjustment proceeds as follows: since the chronologically lastsignal has already been determined, it is easily possible, in threesuccessive steps, to shift the other three, leading signals to thechronologically last signal. The criteria for successful shifting isonce again the average DC voltage component at the output of the phasedetector 14. The chronologically leading signals are compared one afterthe other with the chronologically last signal and the delay time τ ofthe respective leading signal is increased until the average DC voltagecomponent at the output of the phase detector 14 reaches the value zero.This operation is carried out with the aid of the microcontroller 24,the voltage values being compared with a software comparator at 0 volts.The delay times τ_(A), τ_(B), τ_(C), and τ_(D) of the delay elements 7to 10 can in this case be adjusted continuously or in small steps by themicrocontroller 24. When all the signals have been shifted in such a waythat their relative phase angle coincides, the track error signal TE isobtained from the sums of the signals A′+C′ and B′+D′. The automaticadjustment method according to the invention has the advantage ofcompensating for influences on the temporal relationship between thedetector signals, which depends on the disk geometry and the trackgeometry, the type of disk, manufacturing tolerances of the laserscanner, that is to say of the optical device (1 to 5), and also on thelinear speed, that is to say the speed of the optical recording mediumat the location where the light beam is currently situated.

Furthermore, it is possible, with little additional outlay (delayelements 7 to 10, circuit arrangement 11 and low-pass filter 23), withthe inclusion of a logic circuit or, as illustrated, a microcontroller24, to determine the temporal relationship between the detector signalsof the areas A to D and to shift the temporal relationship between thesaid signals by means of the delay elements 7 to 10.

FIG. 2 shows a flow diagram of the method for automatically adjustingthe delay elements 7 to 10 which are able to be set. After the start,all of the delay times τ_(A), τ_(B), τ_(C), and τ_(D) are set to zero.The phase angles of the signals A and B are subsequently compared withone another. The multiplexer is in this case in the switch positionrepresented. If the signal A leads the signal B, then the left-handbranch is taken, otherwise the right-hand branch is taken. In each case,the lagging signal B and A, respectively, is then compared with thesignal C. This corresponds to the fourth and to the second switchposition, respectively, of the circuit arrangement 11 from the top. Thesignal which is determined to be a lagging one in this second comparisonstep is subsequently compared with the signal D.

After this third comparison, it is evident which signal lies thefurthest back in time. In three successive steps, the delay times τ_(i)of the delay elements of the other respective signals are set. On thefar left in FIG. 2, this is illustrated for the case where signal D lagsthe furthest. τ_(A) is first of all adjusted until the signals A′ and Dare in phase, then τ_(B) and, subsequently thereto, τ_(C) arecorrespondingly adjusted. To the right of this, the case where signal Clags the furthest is illustrated. In this case, τ_(A), τ_(B) and τ_(D)are correspondingly adjusted. To the right of this, the case isillustrated where signal B lags the furthest and, on the far right, thecase where signal A lags the furthest. After the other three respectivedelay times τ_(I) have been set, the multiplexer of the circuitarrangement 11 is set to its switch position which is illustrated as thelowermost switch position in the representation. Consequently, thesummation signals A+C and B+D are fed to the phase detector 14 in orderto generate the track error signal TE. The setting operation of thedelay elements 7 to 10 is thus concluded.

What is claimed is:
 1. Device for reading from or writing to opticalrecording media, the device comprising: a detector having four detectorareas for receiving a light beam reflected from an optical recordingmedium; first and second variable delay elements associated withrespective first and second detector areas of the detector; and a phasedetector having inputs respectively associated with outputs of the firstand second variable delay elements and an output that provides a trackerror signal, wherein the first and second variable delay elements canbe set independently of each other.
 2. Device according to claim 1,further comprising a control means for setting one of the first andsecond variable delay elements.
 3. Device according to claim 1, furthercomprising a phase detection means for detecting the phase angles of thesignals emitted by the detector areas of the detector.
 4. Deviceaccording to claim 1, further comprising a circuit arrangement forapplying a variable combination of the output signals of the detectorareas of the detector to a phase angle detector.
 5. Device according toclaim 4, wherein the phase angle detector is the phase detector of thedevice.
 6. Device according to claim 5, further comprising amicrocontroller, which controls the circuit arrangement and the settingof the first and second variable delay elements.
 7. Device according toclaim 5, wherein a low-pass filter is arranged at the output of thephase detector.
 8. Device according to claim 1, characterized in that adelay element is assigned to at least three of the four areas.
 9. Methodfor adjusting variable delay elements, which are able to be set, of adevice for reading from and/or writing to optical recording media, eachvariable delay element being associated with a specific area of aphotodetector of the device, the method comprising the steps of: in afirst method step comparing the phase angle of the output signalsemitted by the variable delay elements associated with the individualareas of the photodetector; and in a second method step altering thedelay times of the variable delay elements until the phases of all theoutput signals coincide with the phase of the chronologically lastoutput signal, and retaining the altered delay times as optimized setvalues of the device.
 10. Method according to claim 9, in which thedelay elements which are able to be set are set to a minimum delay timeprior to the first method step.
 11. Method according to claim 9, inwhich the photodetector comprises areas A, B, C, and D, and the phase ofthe signals of the areas A and B are first of all compared in the firstmethod step and, if the phase of the signal of area A leads that of thearea B, the phases of the signals of the areas B and C are compared,otherwise those of areas A and C are compared; if the phase of thesignal of the area B and A, respectively, leads that of the area C, thenthe phase angles of the signals of the areas C and D are compared withone another, otherwise the phase of the signals of the areas B and A,respectively, is compared with that of the area D, the chronologicallylast signal being evident at the end of these comparisons, the othersignals then being set to the phase angle of the chronologically lastsignal by altering the delay time of the delay elements in accordancewith the second method step.